What's the total value of the world's sunken treasure ?

Three historic shipwrecks have been in the spotlight recently, with cargoes of platinum ingots, gold, and silver estimated at $4.5 billion in total. These huge hauls made us wonder: Just how much money is buried at sea ?

When the last responders leave the sunken cruise liner Costa Concordia, the wreck's status may shift from grave site to treasure trove.

Its passengers and interior decorators reportedly left behind a wealth of cash, jewels, antiques, and thousands of pieces of art. Souvenir hunters, looters, and even the mafia may have plans to dive the wreck for a piece of that fortune.

It's been a busy month for shipwreck headlines and shipwreck hunters. The team that announced the discovery of the Port Nicholson, a World War II–era British merchant ship found 50 miles off the coast of Maine, says it bore 71 tons of platinum ingots worth about $3 billion.

Other shipwreck hunters turned up the HMS Victory, which sank in the English Channel in 1744 with a "secret" cargo of gold valued at $1 billion.

And, in an episode that shows the high stakes of shipwreck salvaging, Spain is currently recovering the estimated $500 million haul of gold and silver from the Spanish galleon Nuestra Señora de las Mercedes that sank in 1804; an American company found the ship but lost court cases to Spain over the rights to the treasure.

All this undersea treasure hunting got us wondering: Just how much money is out there buried at sea? We put the question to marine archeologists, a historian, and a shipwreck hunter. Their answers ranged from "Who knows ?" to "$60 billion"—and each was instructive.

An estimate of the value of sunken treasure in the world begins with a guess at the number of sunken ships. James Delgado, director of the Maritime Heritage Program at the National Oceanic and Atmospheric Administration (NOAA), estimates that there are a million shipwrecks underwater now.

"Given everything that's charted and all the rest, I would say that the majority of them remain undiscovered," Delgado says. After all, 70 percent of the planet's surface is water, and humans have only begun to be able to reach the depths. "[Considering] this, 95 percent of the ocean still remains unknown to us.

It's the last frontier," Delgado says. "We know more about the surface of the moon than what's at the bottom of the sea."

Maritime historian Amy Mitchell-Cook at the University of West Florida says she doesn't think it's possible to make an estimate. "Even in Pensacola Bay, where I am, I don't think we have an accurate number of shipwrecks," she says.

"There were Spanish, French, English, and Americans all in the area, as well as international trade. We know a lot of ships sank, but we don't have a complete set of records."